ext4: add ext4_fc_eligible()

ext4: add ext4_fc_eligible()

Testing EXT4_MF_FC_INELIGIBLE is almost always combined with testing
ext4_fc_disabled().  The code can be simplified by combining these two
in a new ext4_fc_eligible().

In ext4_fc_track_inode() this moves the ext4_fc_disabled() test after
ext4_fc_mark_ineligible(), but as that is a non-op when
ext4_fc_disabled() is true, this is no no consequence.

Note that it is important to still call ext4_fc_mark_ineligible() in
ext4_fc_track_inode() even when ext4_fc_eligible() would return true.
ext4_fc_mark_ineligible() does not ONLY set the "INELIGIBLE" flag but
also updates ->s_fc_ineligible_tid to make sure that the flag remains
set until all ineligible transactions have been committed.

Reviewed-by: Andreas Dilger <adilger@dilger.ca>
Reviewed-by: Jan Kara <jack@suse.cz>
Signed-off-by: NeilBrown <neil@brown.name>
Link: https://patch.msgid.link/20260320000838.3797494-3-neilb@ownmail.net
Signed-off-by: Theodore Ts'o <tytso@mit.edu>