usb: gadget: f_mass_storage: forced_eject attribute

It allows to reset prevent_medium_removal flag and "eject" the image.
This can be useful to free the drive from a hunging host or if the host
continues to use the drive even after unmounting (Linux does this).
It's also a bit like using an unfolded paperclip on an optical drive.

Previously, the undocumented method of sending SIGUSR1 to a special
"file-storage" kernel thread could be used for these purposes,
but when using multiple storages there was no way to distinguish
one from the other, so we had to send a signal to everyone.

Reviewed-by: Alan Stern <stern@rowland.harvard.edu>
Signed-off-by: Maxim Devaev <mdevaev@gmail.com>
Link: https://lore.kernel.org/r/20220711102956.19642-1-mdevaev@gmail.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Documentation/usb/gadget-testing.rst

@@ -333,6 +333,12 @@ In each lun directory there are the following attribute files:
 			being a CD-ROM.
 	nofua		Flag specifying that FUA flag
 			in SCSI WRITE(10,12)
+	forced_eject	This write-only file is useful only when
+			the function is active. It causes the backing
+			file to be forcibly detached from the LUN,
+			regardless of whether the host has allowed it.
+			Any non-zero number of bytes written will
+			result in ejection.
 	=============== ==============================================
 
 Testing the MASS STORAGE function

Documentation/usb/mass-storage.rst

@@ -181,6 +181,15 @@ sysfs entries
     Reflects the state of nofua flag for given logical unit.  It can
     be read and written.
 
+  - forced_eject
+
+    When written into, it causes the backing file to be forcibly
+    detached from the LUN, regardless of whether the host has allowed
+    it.  The content doesn't matter, any non-zero number of bytes
+    written will result in ejection.
+
+    Can not be read.
+
   Other then those, as usual, the values of module parameters can be
   read from /sys/module/g_mass_storage/parameters/* files.