fs: Create anon_inode_getfile_fmode()

Creates an anon_inode_getfile_fmode() function that works similarly to
anon_inode_getfile() with the addition of being able to set the fmode
member.

Signed-off-by: Dawid Osuchowski <linux@osuchow.ski>
Link: https://lore.kernel.org/r/20240426075854.4723-1-linux@osuchow.ski


Signed-off-by: Christian Brauner <brauner@kernel.org>